refactor: add MSat value class

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ovitrif

app/src/main/java/to/bitkit/models/MSat.kt

@@ -0,0 +1,19 @@
+package to.bitkit.models
+
+/**
+ * A wrapper for millisatoshi [ULong] values that provides explicit sat conversions.
+ *
+ * Eliminates scattered `/ 1000u` and `(x + 999u) / 1000u` patterns by encoding
+ * the rounding intent directly in the API: [ceil] rounds up, [floor] truncates.
+ *
+ * Zero runtime overhead — this is an [inline value class][JvmInline].
+ */
+@JvmInline
+value class MSat(val value: ULong) {
+
+    /** Round up to the nearest whole sat. Use for payment/display amounts. */
+    fun ceil(): ULong = (value + 999u) / 1000u
+
+    /** Truncate sub-sat remainder. Use for fees and upper bounds. */
+    fun floor(): ULong = value / 1000u
+}